1. Field of the Invention
The present invention relates to a screen printing plate used for screen process printing.
2. Discussion of Background
There has been widely used a technique of directly printing an electric conductive portion for an electric heater on a glass plate and baking it to prepare a safety glass panel for a vehicle. The electric heater is for the purpose of anti-fogging. The electric conductive portion comprises an element line portion which generates heat for anti-fogging and a bus bar portion for supplying power to the element line portion.
An advantage of the printing technique with use of the screen printing plate is to provide a printed layer having a uniform thickness. Various improvements have been conducted to provide a uniform thickness for the printed layer.
As an anti-fogging method for a safety glass for a vehicle, there has been known a technique that a current is passed through a fine line (an element line) obtained by printing and baking silver paste. There has been also known to use a screen printing method to form the fine line (element line) of silver paste to have a uniform thickness so that heat generated from the element line can be uniform.
In the conventional technique, however, the bus bar portion for supplying power to the element line was apt to generate heat higher than the element line portion because there is a demand of narrowing the width of the bus bar portion in designing the structure of screen printing plates and, therefore, a power density in the bus bar portion is high (the upper limits of temperature in the element line portion and the bus bar portion are determined in Industrial Standards). In order to solve the above-mentioned problem in the screen printing method wherein the thickness of the silver paste layer of the element line portion and the bus bar portion is uniform, the layer thickness of the bus bar portion has to be increased by repeating printing operations two times or more to thereby reduce the power density in the bus bar portion. However, there is a disadvantage that the number of stage of printing (number of times of printing) is increased.
Further, the thickness of the layer of ink is determined by a condition of ink, a condition of printing and a condition of the screen printing plate. There has been known that the thickness of the layer of ink can be changed by adjusting the layer thickness of emulsion and the rate of opening of a gauze when the screen process printing is used. The method of changing the layer thickness of ink by adjusting the layer thickness of emulsion has already been practiced.
In the later method, the condition of the gauze is determined depending on the diameter of yarns constituting the gauze and the density of textile (the number of yarns per inch) which determine the thickness of the gauze and the rate of opening.
Further, Japanese Examined Patent Publication No. 39470/1980 discloses a method of reducing an amount of ink to be applied to a specified area by using a multifilament structure. However, the multifilament structure had problems as follows. When the screen process printing was conducted, cutting or fraying of yarns was resulted. A piece of cut yarn or a fraying yarn adversely influenced the printed surface. Further, ink remained in multifilaments even when the screen printing plate was washed after printing operations, and it was difficult to use continuously the screen printing plate.